dma-mapping: move all DMA mapping code to kernel/dma

Currently the code is split over various files with dma- prefixes in the
lib/ and drives/base directories, and the number of files keeps growing.
Move them into a single directory to keep the code together and remove
the file name prefixes.  To match the irq infrastructure this directory
is placed under the kernel/ directory.

Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/kernel/Makefile b/kernel/Makefile
index d200162..04bc07c 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -41,6 +41,7 @@
 obj-y += irq/
 obj-y += rcu/
 obj-y += livepatch/
+obj-y += dma/
 
 obj-$(CONFIG_CHECKPOINT_RESTORE) += kcmp.o
 obj-$(CONFIG_FREEZER) += freezer.o